1. Field of the Invention
The present invention generally relates to a single laser beam measurement system, and more particularly to such a system having an error correction mechanism which compensates for errors arising from the target not being square to a laser scanning beam.
2. Description of the Related Art
Laser measurement systems or other light-based systems that employ triangulation to determine the position of retro-reflective targets position at specific known reference points, such as on the frame of a vehicle are known such as U.S. Pat. Nos. 4,997,283, 5,251,013 and 5,801,834, the entire disclosures of which are herein incorporated by reference.
In such a system, a laser beam is split into two laser beams by a 50/50 beam splitter, each beam then being directed to a rotating mirror of a laser scanner. The pair of spaced, rotating mirrors direct each laser beam in a 360 degree arc or circle, with both beams being directed in a single plane. The distance between the two laser beams forms the base of the triangle and the two angles generated by the rotating mirrors provide the position of a flat reflective target or targets positioned in the field of measurement.
The system requires each of the rotating mirrors to be “synched” to the mechanical centerline of the laser of the laser scanner which can introduce errors. The synch position is typically calibrated at the factory using a table with a precision grid of targets. There are several factors in addition to synch that are taken into account during this calibration. When the unit is put into the field and measurements taken in situ, it is common for this synch reference to move which causes measurement errors.
In the noted prior art design the two laser beams, spaced at a fixed distance apart (forming the base of the triangle) are swept across a reflective target having a width determined by a reflective stripe on each edge. The center of the two stripes is calculated and the intersection of the two base points across this point allows the position calculation. When using only one scanning laser beam instead of two, there are no longer intersecting points and the position of the target must be determined in a different manner.
The time of the sweep of a single laser beam across the face of the target could be used to determine distance from the laser. However when the target is rotated off perpendicular the width is foreshortened and the target appears to be farther away. This face angle error could be calculated by measuring the leading edge and comparing it to the trailing edge and corrected and compensated for but it is a sine function and very small. Therefore small angular changes of the target lead to large distance errors and it becomes difficult to determine the rotation angle of the target face to the level of accuracy required.